Conventionally, as a construction of the contact point of a switch in a key switch used for, for instance, a cordless telephone set, a carbon electrode pattern is arranged in an interdigitized shape on a printed wiring board, and a conductive material pattern is arranged on the rear surface of a key rubber. The key switch is turned ON/OFF by pressing down the surface of the key rubber.
Also as a material used for a contact point of a key switch, there are a gold-plated contact point and a palladium contact point in addition to a carbon contact point. Such contact points are to be subjected to carbon processing, gold flash processing, and palladium processing respectively in a manufacturing process. However, when these types of processing are viewed from the point of cost, assuming that gold flash processing is 100 [%] in terms of cost, palladium processing is 90 [%] and carbon processing is 60 [%]. For this reason application of a carbon contact point thereto allows a substantial contribution to cost reduction of the products.
It should be noted that, from the view point of reliability, a gold-plated contact point is excellent in reliability, and a carbon contact point is inferior to a gold-plated one. Also a palladium contact point has some problems of migration between palladium and copper foil, accordingly a palladium contact point is slightly inferior in reliability to a gold-plated one.
Also from the view point of small size, for instance, in the case of a substrate having two copper foil layers, a minimum pattern width of a gold-plated contact point and a palladium contact point is 0.2 [mm] which is substantially equivalent to a copper foil pattern. This is in contrast a minimum pattern width of a carbon contact point is 0.4 [mm], which is not suitable for fine working (namely, forming a complicated contact point pattern), and the problem described above has been left unsolved.
Furthermore, there is a demand for a recent cordless telephone set having each key therein expected to be lit individually by a back light so that each of function keys is possible to be identified even in the night or in dark places. For this reason light emitting elements such as a light-emitting diode, are provided in the peripheral section of the key or inside the key region, whereby various types of products responding to the demand have been developed.
In a switch using the conventional type of carbon contact point described above, however, there have been some problems that, for instance, to make a key light individually, it is required to provide light emitting elements inside the key region. Accordingly, the area for the carbon electrode pattern as a contact point of a key switch becomes smaller by the portion equivalent to the area for the light emitting elements as described above. In products requiring size reduction, the key region can not be widened, which resultantly causes imperfect contact on the contact point of a switch, and the reliability thereof is reduced.
Also in this case, although a carbon electrode pattern is to be provided adjacent to the periphery of the light emitting elements, luminous efficiency of the light emitting elements is reduced because carbon is black and absorbs light.